Over the years, numerous methods have been proposed for improving the delivery of biologically active agents, particularly small molecule drugs. Challenges associated with the formulation and delivery of pharmaceutical agents can include poor aqueous solubility of the pharmaceutical agent, toxicity, low bioavailability, instability, and rapid in-vivo degradation, to name just a few. Although many approaches have been devised for improving the delivery of pharmaceutical agents, no single approach is without its drawbacks. For instance, commonly employed drug delivery approaches aimed at solving or at least ameliorating one or more of these challenges include drug encapsulation (such as in a liposome, polymer matrix, or unimolecular micelle), covalent attachment to a water-soluble polymer (i.e., conjugation) such as polyethylene glycol (i.e., PEG or PEGylation), use of gene targeting agents, and the like.
PEGylation has been employed to a limited degree to improve the bioavailability and ease of formulation of small molecule therapeutics having poor aqueous solubilities. For instance, water-soluble polymers such as PEG have been covalently attached to artilinic acid to improve its aqueous solubility. See U.S. Pat. No. 6,461,603. Similarly, PEG has been covalently attached to triazine-based compounds such as trimelamol to improve their solubility in water and enhance their chemical stability. See International Patent Application Publication No. WO 02/043772. Covalent attachment of PEG to bisindolyl maleimides has been employed to improve poor bioavailability of such compounds due to low aqueous solubility. See International Patent Application Publication No. WO 03/037384). Polymer conjugates of non-steroidal anti-inflammatory drugs (NSAIDs) and of opioid antagonists have also been prepared. See U.S. Patent Application Publication Nos. 2007/0025956 and 2006/0105046, respectively. Prodrugs of camptothecin having one or two molecules of camptothecin covalently attached to a linear polyethylene glycol have also been prepared. See U.S. Pat. No. 5,880,131.
Certain drugs, such as the alkaloids, are notoriously difficult to solubilize (i.e., formulate). Such alkaloids include the taxanes, such as docetaxel, and the camptothecins, such as irinotecan. Camptothecin (often abbreviated as “CPT”) is a phytotoxic alkaloid first isolated from the wood and bark of Camptotheca acuminata (Nyssaceae), and has been shown to exhibit antitumor activity. The compound has a pentacyclic ring system with an asymmetric center in lactone ring E with a 20 S configuration. The pentacyclic ring system includes a pyrrolo[3,4-b]quinoline (rings A, B and C), a conjugated pyridone (ring D), and a six-membered lactone (ring E) with a 20-hydroxyl group. Due to its insolubility in water, camptothecin was initially evaluated clinically in the form of a water-soluble carboxylate salt having the lactone ring open to form the sodium salt. The sodium salt, although exhibiting much improved water solubility in comparison to camptothecin itself, produced severe toxicity and demonstrated very little in vivo anticancer activity, thus demonstrating the undesirability of this approach. A Phase I clinical trial for a linear PEG-paclitaxel compound for treatment of patients with advanced solid tumors and lymphomas in 2001; the trial has since been terminated.
In an effort to address the poor aqueous solubility associated with camptothecin and many of its derivatives, a number of synthetic efforts have been directed to derivatizing the A-ring and/or B-ring or esterifying the 20-hydroxyl to improve water-solubility while maintaining cytotoxic activity. For example, topotecan (9-dimethylaminomethyl-10-hydroxy CPT) and irinotecan (7-ethyl-10[4-(1-piperidino)-1-piperidino]carbonyloxy CPT), otherwise known as CPT-11, are two water-soluble CPT derivatives that have shown clinically useful activity. Conjugation of certain camptothecin derivatives, such as 10-hydroxycamptothecin and 11-hydroxycamptothecin, to a linear poly(ethylene glycol) molecule via an ester linkage has been described as a means to form water soluble prodrugs. See U.S. Pat. No. 6,011,042. The approach used relies on reaction of an aromatic, hydroxyl-containing compound with an activated polymer.
The clinical effectiveness of many small molecule therapeutics such as the foregoing, and oncolytics in particular, is limited by several factors such as dose-related toxicity. For instance, irinotecan and other camptothecin derivatives undergo an undesirable hydrolysis of the E-ring lactone under alkaline conditions. Additionally, administration of irinotecan causes a number of troubling side effects, including leukopenia, neutropenia, and diarrhea. Due to its severe diarrheal side-effect, the dose of irinotecan that can be administered in its conventional, unmodified form is extremely limited, thus hampering the efficacy of this drug and others of this type.
These associated side effects, when severe, can be sufficient to arrest further use as well as development of such drugs as promising therapeutics. Additional challenges facing small molecules include high clearance rates, and, with respect to anticancer agents, minimal tumor permeation and residence time. Approaches involving the use of polymer attachment must balance the size of the polymer against the molecular weight of the active agent in order to allow therapeutically effective doses to be delivered, and at a clinically useful rate. Finally, the synthesis of a modified or drug delivery-enhanced active agent must result in reasonable yields, and in a reproducibly prepared product, to make any such approach economically attractive. Thus, there exists a need for new methods for effectively delivering drugs, and in particular small molecule drugs, and even more particularly oncolytics, which can reduce their adverse and often toxic side-effects, whilst simultaneously improving their preparation, efficacy and ease of formulation. Even more importantly, there exists a need to provide oncolytic products that are effective against drug resistant tumors. Specifically, there is a need for improved methods for delivering drugs such as the foregoing that possess an optimal balance of bioavailability due to reduced clearance times, bioactivity, and efficacy, coupled with reduced side-effects.